Liposomes as parenteral drug delivery carriers are currently being utilized in the pharmaceutical industry. There are several FDA-approved liposomal injectable products on the market with many more potential products in clinical trials and in preliminary studies. Liposomes have proven to be useful in cancer, macular degeneration, fungal infections, and vaccines with approved products such as Doxil®, Visudyne QLT®, Ambisome®, and Epaxal®, respectively. Moreover, liposomes as non-viral gene-delivery vectors are being investigated in clinical trials.
To date, there are various types of liposomes that are adapted for different applications such as cancer, gene-delivery, siRNA-delivery, protein/peptide delivery and small molecule delivery. Depending on the application, there will be differences in the liposome formulations such as in lipid type/composition, size and other properties as discussed above. For example, Stealth® liposomes composed of DSPC/Chol/PEG-DSPE are widely used in cancer and cell-targeting applications as a stable liposome formulation due to their prolonged half-life compared to conventional liposomes. For siRNA applications, different fusogenic lipids such as DOPE are used to form complexes with siRNA for efficient delivery.
The properties of liposomes such as the hydrodynamic radius (size), zeta-potential, lipid-packing, encapsulation efficiency, and external modifications (such as polymer coatings) are important in formulating an efficacious drug delivery system. When considering in vivo applications of liposomes, the correct size of liposomes is one property that is vital in order to deliver the liposomes to different locations in the body. For example, liposomes with an approximate diameter of <100 nm are known to accumulate at cancer sites as a result of the enhanced permeability retention (EPR) effect, whereas very small liposomes or larger liposomes are filtered or taken up elsewhere in the body, respectively.
Some liposome properties are highly dependent on the processing conditions of the formulation, and any alterations in these processing conditions will lead to differences in the final formulation. Therefore, it is important to develop a manufacturing system that can accurately and predictively produce liposomes based on the user's requirements.